Recent developments in very high speed integrated circuits (VHSIC) have led to the need for high efficiency, high power density resonant mode and switched mode power supplies, operating at frequencies in excess of 1 Megahertz (MHz). At present, power supply volume exceeds that occupied by all of the electronic components in VHSIC circuitry. Standard core designs and winding constructions result in transformers and inductors which limit the miniaturization process by extending beyond the envelope of the power supply layout. Increasing the operating frequency of the power supply results in reduction in the volume occupied by magnetic components and devices. As switching frequencies are increased, magnetic core losses and electric winding losses are also increased. These developments have placed stringent requirements in ferrite chemical compositions that are capable of providing low magnetic losses at high operating frequencies and magnetic flux densities required for power supplies. Transformers and inductors operating at megahertz frequencies require ferrimagnetic core materials that exhibit low power loss, high as well as low permeability and high dispersion frequency, and adequate Curie temperature.
The power loss density is simply the total power loss of the core material divided by the volume of the core. Typically, power loss density is expressed in units of watts/cm.sup.3, but may also be expressed by the dimensionless parameter known as "loss tangent."
It has long been known in the art that naturally occurring magnetite (Fe.sub.3 O.sub.4) is generally suitable for high frequency applications. Significantly, Fe.sub.3 O.sub.4 has a spinel microstructure. Efforts have therefore focused on developing magnetic ceramics having a similar spinel structure but with enhanced magnetic properties. As is the case with ferrimagnetic materials such as elemental iron, the magnetism of Fe.sub.3 O.sub.4 is attributed to the magnetic moments of the individual ions in the unit cell lattice. For purposes of developing new magnetic materials with enhanced magnetic properties, emphasis is therefore placed upon the various species of ions and their respective location in the lattice and sub-lattice structure. Materials exhibiting the spinel structure may be represented by the formula R.sup.+2 Fe.sub.2.sup.+3 O.sub.4.sup.-2.
In developing ferrimagnetic ceramic materials, practitioners have experimented with ionic substitution to replace Fe ions. For example, displacement of Fe.sup.+3 may be accomplished through inclusion of either magnetic or non-magnetic ions, such as Zn.sup.+2 (0 Bohr Magnetons). In general, ionic substitution of Fe.sub.3 O.sub.4 may be represented by the ionic formula RFe.sub.2.sup.+3 O.sub.4.sup.-2, where R represents various ionic states for Ni, Zn, Mn, Co or other transition metal ions. Thus, ionic substitution with Ni.sup.+2 and Zn.sup.+2 is designated as Ni.sup.+2.sub.1-x Zn.sup.+2.sub.x Fe.sup.+3.sub.2 O.sup.-2.sub.4, where x is the formula coefficient for Zn.
Nickel-zinc-cobalt ferrites have been used for transformer and inductor cores because of their relatively high permeability and low losses. For example, U.S. Pat. Nos. 3,509,058, 3,514,405, 3,533,949 and 3,609,083, which are incorporated herein by reference, each describe methods of producing NiZnCo ferrite core materials for use in high frequency applications. As demonstrated by the prior art, certain NiZnCo ferrites exhibit desirable permeability and reduced power losses at frequencies of up to 20 MHz. Although these advancements in the art are significant, there still exists a need for materials having reduced power loss density in order to improve efficiency and reduce power supply volume using transformer and inductor components operating at ultra high frequencies, in the range of 1-100 MHz, and high flux densities of 1-2500 Gauss.
The tendencies in ferrimagnetic materials for permeability to increase and power loss to decrease as operating frequencies increase have been offset in part by microstructural modification of the core material through specially developed processing techniques, which typically involve one or more high temperature sintering sequences. For example, U.S. Pat. No. 3,609,083, which is incorporated herein by reference, describes a post-sintering heat treatment utilizing rapid cooling from the sintering temperature to a temperature above the Curie temperature. The material is then rapidly quenched and thereafter annealed for a prescribed period of time. U.S. Pat. No. 3,242,089, which is incorporated herein by reference, also describes a heat treatment that incorporates post-sintering annealing. Yet another approach has been to add additional oxide materials to the base composition. For example, U.S. Pat. No. 3,574,116, which is incorporated herein by reference, describes a base composition which includes nominal amounts of SiO.sub.2 and CaO.
Despite the advancements in the art, there still exists a need for optimum NiZnCo ferrite materials that are capable of extending the operating frequency range of such materials in electronic power supply applications.